The vast majority of alkaline ion conductors used in electrochemical cells, especially batteries, are solutions of salts (such as LiCF3SO3 or LiPF6) in either aprotic polar solvents (such as ethylene carbonate) or polar polymers (such as polyethylene oxides). In these environments, dissociation is not complete, i.e., a number of different charged species are present. The effective transference number of Li+ is usually around 30%, and the presence of different kinds of mobile species (e.g., anions, triple ions) leads to significant concentration polarization effects (formation of a salt concentration gradient) at high Li+ currents. Especially for high drain applications such as batteries, a true single ion (in particular Li+) conductor is very desirable since it would allow for higher currents.
Relatively high ionic conductivities were reported for solvated PFSA (perfluorosulfonic acid) ionomers (e.g., Nafion). For the latter, the highest conductivities observed were of the order of 10−3 S/cm at room temperature (U.S. Pat. No. 6,033,804).
PFSA ionomers have been chosen because they were known to show the highest conductivities among all ionomers in their hydrated proton forms. This was thought to be the result of their special microstructure which is distinctly different from this of hydrocarbon based ionomers. This microstructure is thought to be beneficial in obtaining high ionic conductivity, but it is also the reason for the very high hydrodynamic solvent transport (solvent permeation and electroosmotic solvent drag) in such systems. Therefore, non-perfluorinated hydrocarbon ionomers and polyelectrolytes in aprotic media had never been considered as base material in obtaining high alkaline ion conductivity.
Thus, there is a need to provide improved hydrocarbon polymer-based alkaline single ion conductors with very high conductivity and transference number and low hydrodynamic solvent transport in aprotic media and methods for preparing the same.